When a Radio Frequency (RF) transmitter is connected to an antenna by a feed line, the impedance of the antenna and feed line must match exactly for maximum energy transfer from the feed line to the antenna. If an antenna and a feed line do not have matching impedances, some of the electrical energy can not be transferred from the feed line to the antenna. Energy not transferred to the antenna is reflected back towards the transmitter. The interaction of these reflected waves with forward waves causes standing wave patterns. Reflected power in RF transmitters may cause an increase in RF energy losses and damage the RF transmitter.
In RF testing, normally pure resistive 50 ohms loads and cables are used. However, sometimes there is a need for a mismatch, for simulating a non-perfect load situation for a transmitter for instance. Such mismatches should of course be well defined and known.
For some purposes there is a need to adjust the mismatch both in reflection coefficient and phase. For that purpose, it is common to use sliding loads, adjustable stubs and similar equipment. Such equipment needs to be carefully adjusted for each frequency before insertion using a network analyzer.
More commonly, a broadband mismatch, such as a load having an impedance which is not equal to the system impedance may be used. Another approach is to use a through attenuator, leaving the output port either open or shorted. This gives a return loss of approximately twice the attenuators attenuation. Both of the last two methods described above only gives the possibility to control the magnitude of the reflection coefficient. This is often sufficient, for instance when characterizing a VSWR-detector (Voltage Standing Wave Ratio).
A VSWR (voltage standing wave ratio) detector measures the standing wave ratio in a transmission line and is used to check the quality of the match between the antenna and the transmission line.
The above described methods, however, suffer from a flaw that the port of the Device Under Test (DUT) has to be disconnected from other test equipment, for instance a power meter. If RF switch relays are used to switch a in a number of test devices to the DUT, the measurement accuracy will be compromised caused by mismatches and reflections. Positioning the mismatch behind relays and additional cables also gives a very uncertain result and makes an ever so well-defined mismatch device severely unpredictable, from the DUTs point of view.